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Abstract:

A display device including: a pixel array section; power supply lines;
and auxiliary electrodes, wherein each pixel has an auxiliary
capacitance, and one of electrodes of the auxiliary capacitance is
connected to the source electrode of the drive transistor, and another
electrode is connected to the auxiliary electrode for the pixel.

Claims:

1. A display device comprising a pixel array section, the pixel array
section having pixels arranged in a matrix, each of the pixels including
(a) an electro-optical element, (b) a write transistor configured to
provide a video signal, (c) a holding capacitor, and (d) a drive
transistor configured to drive the electro-optical element based on the
video signal, wherein, each pixel has an auxiliary electrode that is
electrically connected to a corresponding power supply line, each pixel
has an auxiliary capacitor, for each pixel, a first electrode of the
auxiliary capacitor is connected to the drive transistor and a second
electrode of the auxiliary capacitor is connected to the auxiliary
electrode, and for each pixel, the auxiliary electrode is formed beneath
the first electrode of the auxiliary capacitor as viewed in cross
section.

2. The display device of claim 1, wherein, in cross section, the second
electrode of the auxiliary capacitor is connected to the auxiliary
electrode via a second wiring layer formed in a layer in which the power
supply line is formed.

3. The display device of claim 2, wherein: the second electrode of the
auxiliary capacitor is connected to the second wiring via a first contact
portion, the second wiring is connected to the auxiliary electrode via a
second contact portion, and the first contact portion and the second
contact portion are overlapped.

4. The display device of claim 1, wherein the second electrode of the
auxiliary capacitor and the second wiring are made of the same metallic
material.

5. The display device of claim 4, wherein: the second electrode of the
auxiliary capacitor is in a wiring layer in which the power supply lines
is located; and the second electrode of the auxiliary capacitor is
opposed to the first electrode of the auxiliary capacitor via an
interlayer insulating film therebetween.

6. The display device of claim 4, wherein: the second electrode of the
auxiliary capacitor is in a wiring in which a gate electrode of the drive
transistor is located, and the second electrode of the auxiliary
capacitor is opposed to the first electrode of the auxiliary capacitor
via a gate insulating film therebetween.

7. The display device of claim 4, wherein: the second electrode of the
auxiliary capacitor comprises a third electrode and a fourth electrode
electrically connected to each other, the third electrode is in a wiring
layer in which gate electrode of the drive transistor is located such
that the third electrode is opposed to the first electrode of the
auxiliary capacitor via a gate insulating film therebetween, and the
fourth electrode is in a wiring layer in which the power supply lines are
located such that the fourth electrode is opposed to the first electrode
of the auxiliary capacitor via an interlayer insulating film
therebetween.

8. Electronic equipment having a display device, the display device
comprising a pixel array section, the pixel array section having pixels
arranged in a matrix, each of the pixels including (a) an electro-optical
element, (b) a write transistor configured to provide a video signal, (c)
a holding capacitor, and (d) a drive transistor configured to drive the
electro-optical element based on the video signal, wherein, each pixel
has an auxiliary electrode that is electrically connected to a
corresponding power supply line, each pixel has an auxiliary capacitor,
for each pixel, a first electrode of the auxiliary capacitor is connected
to the drive transistor and a second electrode of the auxiliary capacitor
is connected to the auxiliary electrode, and for each pixel, the
auxiliary electrode is formed beneath the first electrode of the
auxiliary capacitor as viewed in cross section.

Description:

CROSS REFERENCES TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent application Ser.
No. 12/190,366 filed Aug. 12, 2008, the entirety of which is incorporated
herein by reference to the extent permitted by law. The present
application also claims priority to Japanese Patent Application JP
2007-211623 filed in the Japan Patent Office on Aug. 15, 2007, the entire
contents of which being incorporated herein by reference to the extent
permitted by law.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a display device and electronic
equipment, and more particularly to a flat panel display device and
electronic equipment having the same in which pixels, each incorporating
an electro-optical element, are disposed in a matrix form.

[0004] 2. Description of the Related Art

[0005] In the field of image display device, flat panel display devices
having pixels (pixel circuits), each incorporating an electro-optical
element, disposed in a matrix form, are rapidly becoming widespread.
Among flat panel display devices, the development and commercialization
of organic EL display devices using organic EL (Electro Luminescence)
elements have been continuing at a steady pace. An organic EL element is
a type of current-driven electro-optical element whose light emission
brightness changes according to the current flowing through the element.
This type of element relies on the phenomenon that an organic thin film
emits light when applied with an electric field.

[0006] An organic EL display device has the following features. That is,
it is low in power consumption because organic EL elements can be driven
by a voltage of 10V or less. Besides, organic EL elements are
self-luminous. Therefore, an organic EL display device offers higher
image visibility as compared to a liquid crystal display device designed
to display an image by controlling the light intensity from the light
source (backlight) for each of the pixels containing liquid crystal
cells. Further, an organic EL display device desires no lighting members
such as backlight as desired for a liquid crystal display device, thus
making it easier to reduce weight and thickness. Still further, organic
EL elements are extremely fast in response speed or several μ seconds
or so. This provides a moving image free from afterimage.

[0007] An organic EL display device can be either simple (passive)-matrix
or active-matrix driven as with a liquid crystal display device. It
should be noted, however, that a simple matrix display device has some
problems although simple in construction. Such problems include
difficulty in implementing a large high-definition display device because
the light emission period of the electro-optical elements diminishes with
increase in the number of scan lines (i.e., number of pixels).

[0008] For this reason, the development of active matrix display devices
has been going on at a brisk pace in recent years. Such display devices
control the current flowing through the electro-optical element with an
active element such as insulating gate field effect transistor
(typically, thin film transistor or TFT) provided in the same pixel
circuit as the electro-optical element. In an active matrix display
device, the electro-optical elements maintain light emission over a frame
interval. As a result, a large high-definition display device can be
implemented with ease.

[0009] Incidentally, the I-V characteristic (current-voltage
characteristic) of the organic EL element is typically known to
deteriorate over time (so-called deterioration over time). In a pixel
circuit using an N-channel TFT as a transistor adapted to current-drive
the organic EL element (hereinafter written as "drive transistor"), the
organic EL element is connected to the source of the drive transistor.
Therefore, if the I-V characteristic of the organic EL element
deteriorates over time, a gate-to-source voltage Vgs of the drive
transistor changes, thus changing the light emission brightness of the
same element.

[0010] This will be described more specifically below. The source
potential of the drive transistor is determined by the operating point
between the drive transistor and organic EL element. If the I-V
characteristic of the organic EL element deteriorates, the operating
point between the drive transistor and organic EL element will change. As
a result, the same voltage applied to the gate of the drive transistor
changes the source potential of the drive transistor. This changes the
gate-to-source voltage Vgs of the drive transistor, thus changing the
current level flowing through the drive transistor. Therefore, the
current level flowing through the organic EL element also changes. As a
result, the light emission brightness of the organic EL element changes.

[0011] In a pixel circuit using a polysilicon TFT, on the other hand, a
threshold voltage Vth of the drive transistor or a mobility μ of a
semiconductor thin film making up the channel of the drive transistor
(hereinafter written as "mobility of the drive transistor") changes over
time or is different from one pixel to another due to the manufacturing
process variation (the transistors have different characteristics), in
addition to the deterioration of the I-V characteristic over time.

[0012] If the threshold voltage Vth or mobility μ of the drive
transistor is different from one pixel to another, the current level
flowing through the drive transistor varies from one pixel to another.
Therefore, the same voltage applied to the gates of the drive transistors
leads to a difference in light emission brightness of the organic EL
element between the pixels, thus impairing the screen uniformity.

[0013] Therefore, the compensation and correction functions are provided
in each of the pixels to ensure immunity to deterioration of the I-V
characteristic of the organic EL element over time and variation in the
threshold voltage Vth or mobility μ of the drive transistor over time,
thus maintaining the light emission brightness of the organic EL element
constant (refer, for example, to Japanese Patent Laid-Open No.
2006-133542 (hereinafter referred to as Patent Document 1)). The
compensation function compensates for the variation in characteristic of
the organic EL element. One of the correction functions corrects the
variation in the threshold voltage Vth of the drive transistor
(hereinafter written as "threshold correction"). Another correction
function corrects the variation in the mobility μ of the drive
transistor (hereinafter written as "mobility correction").

SUMMARY OF THE INVENTION

[0014] In the related art described in Patent Document 1, the compensation
function adapted to compensate for the variation in the characteristic of
the organic EL element and the correction functions adapted to correct
the variation in the threshold voltage Vth and mobility μ are provided
in each of the pixels. This ensures immunity to deterioration of the I-V
characteristic of the organic EL element over time and variation in the
threshold voltage Vth or mobility μ of the drive transistor over time,
thus maintaining the light emission brightness of the organic EL element
constant. However, the related art desires a number of elements to make
up each pixel, thus causing an impediment to reducing the pixel size and,
by extension, providing a higher-definition display device.

[0015] On the other hand, a write gain for writing a video signal to the
pixel is determined by factors such as the capacitance value of a holding
capacitance adapted to hold the written video signal and the capacitive
component of the organic EL element (the details thereof will be
described later). As display devices grow in definition, the pixel size
becomes finer. As a result, the electrodes making up the organic EL
element become smaller. Accordingly, the capacitance value of the
capacitive component of the organic EL element is smaller, thus resulting
in a lower video signal write gain. If the write gain declines, a signal
potential appropriate to the video signal may not be held in the holding
capacitance. As a result, the light emission brightness appropriate to
the video signal level may not be achieved.

[0016] In light of the foregoing, it is a purpose of the embodiment of the
present invention to provide a display device and electronic equipment
having the same, each of whose pixels is made up of fewer components and
which can secure a sufficient video signal write gain.

[0017] In order to achieve the above desire, the display device according
to the embodiment of the present invention is defined in that it includes
a pixel array section, power supply lines and auxiliary electrodes. The
pixel array section includes pixels arranged in a matrix form. Each of
the pixels includes an electro-optical element and write transistor
adapted to write a video signal and holding capacitance adapted to hold
the video signal written by the write transistor. Each of the pixels
further includes a drive transistor adapted to drive the electro-optical
element based on the video signal held by the holding capacitance. The
power supply lines are disposed one for each of the pixel rows of the
pixel array section and in the proximity of the scan line which belongs
to the adjacent pixel row. The power supply lines selectively apply a
first potential and a second potential lower than the first potential to
the drain electrode of the drive transistor. The auxiliary electrodes are
disposed in rows, in columns or in a grid form for the pixels of the
pixel array section arranged in a matrix form. The auxiliary electrodes
are applied with a fixed potential. The pixels each have an auxiliary
capacitance. One of the electrodes of the auxiliary capacitance is
connected to the source electrode of the drive transistor. The other
electrode thereof is connected to the auxiliary electrode for each pixel.

[0018] In the display device configured as described above and electronic
equipment having the same, the first and second potentials are
selectively applied to the drain electrode of the drive transistor via
the power supply line. The drive transistor supplied with a current from
the power supply line drives the electro-optical element to emit light
when supplied with the first potential. The same transistor does not
drive the electro-optical element to emit light when supplied with the
second potential. As a result, the drive transistor has the capabilities
to control the light emission and non-light emission of the same element
as well as current-drive the electro-optical element. This eliminates the
need for a transistor adapted specifically to control the light emission
and non-light emission.

[0019] Further, the auxiliary capacitance, one of whose ends is connected
to the source electrode of the drive transistor, makes it possible to
increase the video signal write gain by the capacitance value of the
auxiliary capacitance because the gain is determined by the capacitance
values of the capacitive component of the electro-optical element and the
holding and auxiliary capacitances. Here, the auxiliary electrodes, which
are disposed in rows, in columns or in a grid form for the pixels of the
pixel array section arranged in a matrix form and which are applied with
a fixed potential, are each connected to one of the electrodes of the
auxiliary capacitance for each pixel. This makes it possible to apply a
fixed potential to the other electrode of the auxiliary capacitance
without providing any cathode wiring in a TFT layer, thus allowing to
form the auxiliary capacitance for the fixed potential.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 is a system configuration diagram illustrating the schematic
configuration of an active matrix organic EL display device which is a
prerequisite for the embodiment of the present invention;

[0021]FIG. 2 is a circuit diagram illustrating a specific example of the
configuration of a pixel (pixel circuit);

[0022] FIG. 3 is a timing waveform diagram used for the description of the
operation of the active matrix organic EL display device which is a
prerequisite for the embodiment of the present invention;

[0023] FIGS. 4A to 4D are explanatory diagrams (1) illustrating the
circuit operation of the active matrix organic EL display device which is
a prerequisite for the embodiment of the present invention;

[0024] FIGS. 5A to 5D are explanatory diagrams (2) illustrating the
circuit operation of the active matrix organic EL display device which is
a prerequisite for the embodiment of the present invention;

[0025] FIGS. 6A to 6C are explanatory diagrams (3) illustrating the
circuit operation of the active matrix organic EL display device which is
a prerequisite for the embodiment of the present invention;

[0026] FIG. 7 is a characteristic diagram used for the description of the
problem caused by the variation of a threshold voltage Vth of a drive
transistor;

[0027] FIG. 8 is a characteristic diagram used for the description of the
problem caused by the variation of a mobility μ of a drive transistor;

[0028] FIGS. 9A to 9C are characteristic diagrams used for the description
of the relationship between a video signal voltage Vsig and a
drain-to-source current Ids of the drive transistor with and without the
threshold and mobility corrections;

[0029] FIG. 10 is a circuit diagram illustrating the pixel configuration
having an auxiliary capacitance;

[0030] FIG. 11 is an equivalent circuit diagram illustrating a wiring
resistance R resulting from a cathode wiring run in a TFT layer;

[0031]FIG. 12 is a timing waveform diagram illustrating the variation of
a cathode potential caused by the wiring resistance R;

[0035] FIG. 16 is a sectional view illustrating the sectional structure of
the pixel according to example 1;

[0036] FIG. 17 is a sectional view illustrating the sectional structure of
the pixel according to example 2;

[0037] FIG. 18 is a sectional view illustrating the sectional structure of
the pixel according to example 3;

[0038] FIG. 19 is a perspective view illustrating the appearance of a
television set to which the embodiment of the present invention is
applied;

[0039] FIGS. 20A and 20B are perspective views illustrating the appearance
of a digital camera to which the embodiment of the present invention is
applied, and FIG. 20A is a perspective view as seen from the front, and
FIG. 20B is a perspective view as seen from the rear;

[0040] FIG. 21 is a perspective view illustrating the appearance of a
laptop personal computer to which the embodiment of the present invention
is applied;

[0041]FIG. 22 is a perspective view illustrating the appearance of a
video camcorder to which the embodiment of the present invention is
applied; and

[0042] FIGS. 23A to 23G are external views illustrating a mobile phone to
which the embodiment of the present invention is applied, and FIG. 23A is
a front view of the mobile phone in an open position, FIG. 23B is a side
view thereof, FIG. 23C is a front view thereof in a closed position, FIG.
23D is a left side view thereof, FIG. 23E is a right side view thereof,
FIG. 23F is a top view thereof, and FIG. 23G is a bottom view thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] The embodiments of the present invention provide the drive
transistor with the capabilities to control the light emission and
non-light emission of the same element as well as current-drive the
electro-optical element. This makes it possible to make up each pixel
with fewer components, i.e., merely the write and drive transistors. At
the same time, a sufficient video signal write gain can be secured by
providing the auxiliary capacitance in addition to the holding
capacitance.

[0044] Further, the other electrode of the auxiliary capacitance is
connected, for each pixel, to one of the auxiliary electrodes which are
disposed in rows, in columns or in a grid form for the pixels of the
pixel array section arranged in a matrix form. This makes it possible to
apply a fixed potential to the other electrode without providing any
cathode wiring in the TFT layer. As a result, the auxiliary capacitance
can be formed for the fixed potential while at the same time suppressing
the wiring resistance. This suppresses horizontal crosstalk caused by the
wiring resistance, thus providing improved on-screen image quality.

[0045] A detailed description will be given below of the preferred
embodiment of the present invention with reference to the accompanying
drawings.

[Display Device as a Prerequisite for the Present Invention]

[0046] FIG. 1 is a system configuration diagram illustrating the schematic
configuration of an active matrix display device which is a prerequisite
for the embodiment of the present invention.

[0047] Here, a description will be given taking, as an example, an active
matrix organic EL display device. The organic EL display device uses, as
a light emitting element of each of the pixels (pixel circuits), an
organic EL element (organic electroluminescent element) which is a
current-driven electro-optical element whose light emission brightness
changes according to the current flowing through the element.

[0048] As illustrated in FIG. 1, an organic EL display device 10 includes
a pixel array section 30 and driving sections. The pixel array section 30
has pixels (PXLCs) 20 arranged two-dimensionally in a matrix form. The
driving sections are disposed around the pixel array section 30 and
adapted to drive the pixels 20. Among the driving sections adapted to
drive the pixels 20 are a write scan circuit 40, power supply scan
circuit 50 and horizontal drive circuit 60.

[0049] The pixel array section 30 has one of scan lines 31-1 to 31-m and
one of power supply lines 32-1 to 32-m disposed for each pixel row and
one of signal lines 33-1 to 33-n disposed for each pixel column for the
pixels arranged in m rows by n columns.

[0050] The pixel array section 30 is typically formed on a transparent
insulating substrate such as glass substrate to provide a flat panel
structure. The pixels 20 of the pixel array section 30 may be formed with
amorphous silicon TFTs (Thin Film Transistors) or low-temperature
polysilicon TFTs. When low-temperature polysilicon TFTs are used, the
write scan circuit 40, power supply scan circuit 50 and horizontal drive
circuit 60 can also be implemented on a display panel (substrate) 70 on
which the pixel array section 30 is formed.

[0051] The write scan circuit 40 includes shift registers or other
components adapted to sequentially shift (transmit) a start pulse sp in
synchronism with a clock pulse ck. During the writing of a video signal
to the pixels 20 of the pixel array section 30, the same circuit 40
sequentially supplies write pulses WS1 to WSm (scan signals) respectively
to the scan lines 31-1 to 31-m so as to scan the pixels 20 of the pixel
array section 30 in succession on a row-by-row basis (progressive scan).

[0052] The power supply scan circuit 50 includes shift registers or other
components adapted to sequentially shift (transmit) the start pulse sp in
synchronism with the clock pulse ck. The same circuit 50 sequentially and
selectively supplies power supply line potentials DS1 to DSm respectively
to the power supply lines 32-1 to 32-m in synchronism with the
progressive scan by the write scan circuit 40 so as to control the light
emission and non-light emission of the pixels 20. The power supply line
potentials DS1 to DSm are each switched between two different potentials,
i.e., a first potential Vccp and a second potential Vini lower than the
first potential Vccp.

[0053] The horizontal drive circuit 60 selects, as appropriate, either a
video signal voltage Vsig (hereinafter may be simply written as "signal
voltage") appropriate to the brightness information or an offset voltage
Vofs supplied from a signal supply source (not shown) so as to, for
example, write the selected voltage to the pixels 20 of the pixel array
section 30 via the signal lines 33-1 to 33-n on a row-by-row basis. That
is, the horizontal drive circuit 60 employs progressive writing adapted
to sequentially write the video signal voltage Vsig on a row-by-row
(line-by-line) basis.

[0054] Here, the offset voltage Vofs is a reference voltage (e.g., voltage
corresponding to the black level) which serves as a reference for the
video signal voltage Vsig. On the other hand, the second potential Vini
is set to a potential lower than the offset voltage Vofs. For example,
letting the threshold voltage of the drive transistor 22 be denoted by
Vth, the second potential Vini is set to a potential lower than Vofs-Vth,
and preferably to a potential sufficiently lower than Vofs-Vth.

(Pixel Circuit)

[0055]FIG. 2 is a circuit diagram illustrating a specific example of the
configuration of the pixel (pixel circuit) 20.

[0056] As illustrated in FIG. 2, the pixel 20 includes, for example, as a
light emitting element, an organic EL element 21 which is a type of
current-driven electro-optical element whose light emission brightness
changes according to the current flowing through the element. In addition
to the same element 21, the pixel 20 includes a drive transistor 22,
write transistor 23 and holding capacitance 24 as its components. That
is, the pixel 20 is made up of two transistors (Tr) and one capacitor
(C).

[0057] In the pixel 20 configured as described above, N-channel TFTs are
used as the drive transistor 22 and write transistor 23. It should be
noted, however, that the combination of conductivity types of the drive
transistor 22 and write transistor 23 given here is merely an example,
and the embodiment of the present invention is not limited to this
combination.

[0058] The organic EL element 21 has its cathode electrode connected to a
common power supply line 34 which is disposed commonly for all the pixels
20. The drive transistor 22 has its source electrode connected to the
anode electrode of the organic EL element 21 and its drain electrode
connected to the power supply line 32 (one of 32-1 to 32-m).

[0059] The write transistor 23 has its gate electrode connected to the
scan line 31 (one of 31-1 to 31-m). The same transistor 23 has one of the
source and drain electrodes connected to the signal line 33 (one of 33-1
to 33-n) and the other of the source and drain electrodes connected to
the gate electrode of the drive transistor 22.

[0060] The holding capacitance 24 has one of its electrodes connected to
the gate electrode of the drive transistor 22. The same capacitance 24
has its other electrode connected to the source electrode of the drive
transistor 22 (anode electrode of the organic EL element 21).

[0061] In the pixel 20 made up of two transistors and one capacitor, the
write transistor 23 conducts in response to the scan signal applied to
its gate electrode by the write scan circuit 40 via the scan line 31. As
the same transistor 23 conducts, it samples either the video signal
voltage Vsig appropriate to the brightness information or offset voltage
Vofs supplied from the horizontal drive circuit 60 via the signal line 33
and writes the sampled voltage to the pixel 20.

[0062] The written signal voltage Vsig or offset voltage Vofs is applied
to the gate electrode of the drive transistor 22 and at the same time
held by the holding capacitance 24. When the potential DS of the power
supply line 32 (one of 32-1 to 32-m) is at the first potential Vccp, the
drive transistor 22 is supplied with a current from the power supply line
32. As a result, the drive transistor 22 supplies the organic EL element
with a drive current whose level is appropriate to the voltage level of
the signal voltage Vsig held by the holding capacitance 24, thus
current-driving the same element 21 to emit light.

(Circuit Operation of the Organic EL Display Device)

[0063] A description will be given next of the circuit operation of the
organic EL display device 10 configured as described above based on the
timing waveform diagram shown in FIG. 3 and using the operation
explanatory diagrams shown in FIGS. 4 to 6. It should be noted that the
write transistor 23 is represented by a switch symbol for simplification
in the operation explanatory diagrams shown in FIGS. 4 to 6. It should
also be noted that because the organic EL element 21 has a capacitive
component, an EL capacitance 25 thereof is also shown.

[0064] The timing waveform diagram in FIG. 3 illustrates the variations of
the potential (write pulse) WS of the scan line 31 (one of 31-1 to 31-m),
potential DS (Vccp/Vini) of the power supply line 32 (one of 32-1 to
32-m) and gate potential Vg and source potential Vs of the drive
transistor 22.

<Light Emission Period>

[0065] In the timing diagram shown in FIG. 3, the organic EL element 21
emits light prior to time t1 (light emission period). In the light
emission period, the potential DS of the power supply line 32 is at the
first potential Vccp, and the write transistor 23 is not conducting.

[0066] At this time, because the drive transistor 22 is designed to
operate in the saturation region, a drive current (drain-to-source
current) Ids appropriate to the gate-to-source voltage Vgs of the drive
transistor 22 is supplied to the organic EL element 21 from the power
supply line 32 via the drive transistor 22 as illustrated in FIG. 4A. As
a result, the organic EL element 21 emits light at the brightness
appropriate to the level of the drive current Ids.

<Preparatory Period for Threshold Correction>

[0067] Then, at time t1, the progressive scan of a new field begins. The
potential DS of the power supply line 32 changes from the first potential
(hereinafter written as "high potential") Vccp to the second potential
(hereinafter written as "low potential") Vini which is sufficiently lower
than Vofs-Vth (Vofs: offset voltage of the signal line 33).

[0068] Here, letting the threshold voltage of the organic EL element 21 be
denoted by Vel and the potential of the common power supply line 34 by
Vcath and assuming that Vini<Vel+Vcath for the low potential Vini, the
source potential Vs of the drive transistor 22 is almost equal to the low
potential Vini. As a result, the organic EL element 21 is reverse-biased,
causing it to stop emitting light.

[0069] Next, at time t2, the potential WS of the scan line 31 changes from
the low to high potential, bringing the write transistor 23 into
conduction as illustrated in FIG. 4C. At this time, the horizontal drive
circuit 60 supplies the offset voltage Vofs to the signal line 33.
Therefore, the gate potential Vg of the drive transistor 22 becomes equal
to the offset voltage Vofs. Further, the source potential Vs of the drive
transistor 22 is at the low potential Vini which is sufficiently lower
than the offset voltage Vofs.

[0070] At this time, the gate-to-source voltage Vgs of the drive
transistor 22 is Vofs-Vini. Here, the threshold correction operation may
not be performed unless Vofs-Vini is larger than the threshold voltage
Vth of the drive transistor 22. Therefore, the potential relationship
Vofs-Vini>Vth have to be established. Thus, the preparatory operation
for threshold correction includes of fixing the gate potential Vg and
source potential Vs of the drive transistor 22 respectively to the offset
voltage Vofs and low potential Vini for initialization.

<First Threshold Correction Period>

[0071] Next, at time t3, as the potential DS of the power supply line 32
changes from the low potential Vini to the high potential Vccp as
illustrated in FIG. 4D, the source potential Vs of the drive transistor
22 begins to rise, initiating the first threshold correction period. In
the first threshold correction period, as the source potential Vs of the
drive transistor 22 rises, the gate-to-source voltage Vgs of the drive
transistor 22 reaches a given potential Vx1. The potential Vx1 is held by
the holding capacitance 24.

[0072] Next, at time t4 in the second half of the horizontal interval
(1H), the horizontal drive circuit 60 supplies the video signal voltage
Vsig to the signal line 33 as illustrated in FIG. 5A, changing the
potential of the signal line 33 from the offset voltage Vofs to the
signal voltage Vsig. In this period, the signal voltage Vsig is written
to the pixels in other row.

[0073] At this time, in order to prevent the signal voltage Vsig from
being written to the pixels in the own row, the potential WS of the scan
line 31 changes from the high to low potential, bringing the write
transistor 23 out of conduction. This disconnects the gate electrode of
the drive transistor 22 from the signal line 33, leaving the gate
electrode floating.

[0074] Here, if the gate electrode of the drive transistor 22 is floating
and if the source potential Vs of the drive transistor 22 varies due to
the connection of the holding capacitance 24 between the gate and source
electrodes of the drive transistor 22, the gate potential Vg of the same
transistor 22 also varies with variation (varies to follow the variation)
in the source potential Vs. This is the bootstrapping action by the
holding capacitance 24.

[0075] At time t4 and beyond, the source potential Vs of the drive
transistor 22 continues to rise by Va1 (Vs=Vofs-Vx1+Va1). At this time,
the gate potential Vg of the drive transistor 22 also rises by Va1
(Vg=Vofs+Va1) with the rise of the source potential Vs of the same
transistor 22 because of the bootstrapping action.

<Second Threshold Correction Period>

[0076] At time t5, a next horizontal interval begins. As illustrated in
FIG. 5B, the potential WS of the scan line 31 changes from the low to
high potential, bringing the write transistor 23 into conduction. At the
same time, the horizontal drive circuit 60 supplies the offset voltage
Vofs, rather than the signal voltage Vsig, to the signal line 33,
initiating the second threshold correction period.

[0077] In the second threshold correction period, as the write transistor
23 conducts, the offset voltage Vofs is written. Therefore, the gate
potential Vg of the drive transistor 22 is initialized again to the
offset voltage Vofs. The source potential Vs declines with the decline of
the gate potential Vg at this time. Then, the source potential Vs of the
drive transistor 22 begins to rise again.

[0078] Then, as the source potential Vs of the drive transistor 22 rises
in the second threshold correction period, the gate-to-source voltage Vgs
of the same transistor 22 reaches a given potential Vx2. The potential
Vx2 is held by the holding capacitance 24.

[0079] Next, at time t6 in the second half of the horizontal interval, the
horizontal drive circuit 60 supplies the signal voltage Vsig to the
signal line 33 as illustrated in FIG. 5C, changing the potential of the
signal line 33 from the offset voltage Vofs to the signal voltage Vsig.
In this period, the signal voltage Vsig is written to the pixels in other
row (row next to the row in which the pixels were written the last time).

[0080] At this time, in order to prevent the signal voltage Vsig from
being written to the pixels in the own row, the potential WS of the scan
line 31 changes from the high to low potential, bringing the write
transistor 23 out of conduction. This disconnects the gate electrode of
the drive transistor 22 from the signal line 33, leaving the gate
electrode floating.

[0081] At time t6 and beyond, the source potential Vs of the drive
transistor 22 continues to rise by Va2 (Vs=Vofs-Vx1+Va2). At this time,
the gate potential Vg of the drive transistor 22 also rises by Va2
(Vg=Vofs+Va2) with the rise of the source potential Vs of the same
transistor 22 because of the bootstrapping action.

<Third Threshold Correction Period>

[0082] At time t7, a next horizontal interval begins. As illustrated in
FIG. 5D, the potential WS of the scan line 31 changes from the low to
high potential, bringing the write transistor 23 into conduction. At the
same time, the horizontal drive circuit 60 supplies the offset voltage
Vofs, rather than the signal voltage Vsig, to the signal line 33,
initiating the third threshold correction period.

[0083] In the third threshold correction period, as the write transistor
23 conducts, the offset voltage Vofs is written. Therefore, the gate
potential Vg of the drive transistor 22 is initialized again to the
offset voltage Vofs. The source potential Vs declines with the decline of
the gate potential Vg at this time. Then, the source potential Vs of the
drive transistor 22 begins to rise again.

[0084] As the source potential Vs of the drive transistor 22 rises, the
gate-to-source voltage Vgs of the same transistor 22 will converge to the
threshold voltage Vth of the same transistor 22 before long. As a result,
the voltage corresponding to the threshold voltage Vth is held by the
holding capacitance 24.

[0085] As a result of the third threshold correction operation described
above, the threshold voltage Vth of the drive transistor 22 in each of
the pixels is detected, and the voltage corresponding to the threshold
voltage Vth held by the holding capacitance 24. It should be noted that,
in the third threshold correction period, the potential Vcath of the
common power supply line 34 is set so that the organic EL element 21 goes
into cutoff. This is done to ensure that a current flows merely to the
holding capacitance 24 and not to the organic EL element 21.

<Signal Write Period and Mobility Correction Period>

[0086] Next, at time t8, the potential WS of the scan line 31 changes to
the low potential, bringing the write transistor 23 out of conduction as
illustrated in FIG. 6A. At the same time, the potential of the signal
line 33 changes from the offset voltage Vofs to the video signal voltage
Vsig.

[0087] As the write transistor 23 stops conducting, the gate electrode of
the drive transistor 22 is left floating. However, the gate-to-source
voltage Vgs of the drive transistor 22 is equal to the threshold voltage
Vth of the same transistor 22. Therefore, the same transistor 22 is in
cutoff. As a result, the drain-to-source current Ids does not flow
through the drive transistor 22.

[0088] Next, at time t9, the potential WS of the scan line 31 changes to
the high potential, bringing the write transistor 23 into conduction as
illustrated in FIG. 6B. As a result, the same transistor 23 samples the
video signal voltage Vsig and writes the voltage to the pixel 20. This
writing of the signal voltage Vsig by the write transistor 23 brings the
gate potential Vg of the drive transistor 22 equal to the signal voltage
Vsig.

[0089] Then, when the drive transistor 22 drives the organic EL element 21
with the video signal voltage Vsig, the threshold voltage Vth of the
drive transistor 22 is cancelled by the voltage held by the holding
capacitance 24 which corresponds to the threshold voltage Vth, thus
achieving the threshold correction. The principle of the threshold
correction will be described later.

[0090] At this time, the organic EL element 21 is in cutoff (high
impedance state) at first. Therefore, the current flowing from the power
supply line 32 to the drive transistor 22 according to the video signal
voltage Vsig (drain-to-source current Ids) flows into the EL capacitance
25 of the organic EL element 21, thus initiating the charging of the same
capacitance 25.

[0091] Because of the charging of the EL capacitance 25, the source
potential Vs of the drive transistor 22 rises over time. At this time,
the variation of the threshold voltage Vth of the drive transistor 22 has
already been corrected (by the threshold correction). As a result, the
drain-to-source current Ids of the drive transistor 22 is dependent
merely upon the mobility μ of the same transistor 22.

[0092] When the source potential Vs of the drive transistor 22 rises to
the potential equal to Vofs-Vth+ΔV before long, the gate-to-source
voltage Vgs of the same transistor 22 becomes equal to
Vsig-Vofs+Vth-ΔV. That is, the increment ΔV of the source
potential Vs acts so that it is subtracted from the voltage
(Vsig-Vofs+Vth) held by the holding capacitance 24, in other words, so
that the charge stored in the holding capacitance 24 is discharged. This
means that a negative feedback is applied. Therefore, the increment
ΔV of the source potential Vs of the drive transistor 22 is a
feedback amount of the negative feedback.

[0093] As described above, if the drain-to-source current Ids flowing
through the drive transistor 22 is negatively fed back to the gate input,
i.e., the gate-to-source voltage Vgs, of the same transistor 22, the
dependence of the drain-to-source current Ids of the same transistor 22
upon the mobility μ can be cancelled. That is, the variation of the
mobility μ between the pixels can be corrected.

[0094] More specifically, the higher the video signal voltage Vsig, the
larger the drain-to-source current Ids, and therefore the larger the
absolute value of the negative feedback amount (correction amount)
ΔV. As a result, the mobility is corrected according to the light
emission brightness. If the video signal voltage Vsig is maintained
constant, the larger the mobility μ of the drive transistor 22, the
larger the absolute value of the negative feedback amount ΔV. This
makes it possible to eliminate the variation of the mobility μ between
the pixels. The principle of the mobility correction will be described
later.

<Light Emission Period>

[0095] Next, at time t10, the potential WS of the scan line 31 changes to
the low potential, bringing the write transistor 23 out of conduction as
illustrated in FIG. 6C. This disconnects the gate electrode of the drive
transistor 22 from the signal line 33, leaving the gate electrode
floating.

[0096] When the gate electrode of the drive transistor 22 is left floating
and at the same time the drain-to-source current Ids of the same
transistor 22 begins to flow into the organic EL element 21, the anode
potential of the same element 21 rises according to the drain-to-source
current Ids of the same transistor 22.

[0097] The rise of the anode potential of the organic EL element 21 is
nothing other than the rise of the source potential Vs of the drive
transistor 22. As the source potential Vs of the drive transistor 22
rises, the gate potential Vg of the same transistor 22 will also rise
because of the bootstrapping action.

[0098] At this time, assuming that the bootstrap gain is unity (ideal
value), the increment of the gate potential Vg is equal to the increment
of the source potential Vs. In the light emission period, therefore, the
gate-to-source voltage Vgs of the drive transistor 22 is maintained
constant at Vsig-Vofs+Vth-ΔV. Then, at time t11, the potential of
the signal line 33 changes from the video signal voltage Vsig to the
offset voltage Vofs.

[0099] As is clear from the above description of the operation, the
threshold correction period spans three horizontal intervals, i.e., one
horizontal interval during which the signal writing and mobility
correction are performed and two horizontal intervals preceding the one
horizontal interval. This provides a sufficient time for the threshold
correction period, thus allowing to reliably detect the threshold voltage
Vth of the drive transistor 22 and hold the voltage in the holding
capacitance 24 for the reliable threshold correction operation.

[0100] Although the threshold correction period spans three horizontal
intervals, this is merely an example. If the one horizontal interval
during which the signal writing and mobility correction are performed is
sufficient for the threshold correction period, there is no need to
provide a threshold correction period spanning the preceding horizontal
intervals. On the other hand, if one horizontal interval becomes shorter
as a result of providing a higher definition and if three horizontal
intervals are not sufficient for the threshold correction period, this
period may span four horizontal intervals or longer.

(Principle of the Threshold Correction)

[0101] Here, a description will be given of the principle of the threshold
correction of the drive transistor 22. The drive transistor 22 is
designed to operate in the saturation region. Therefore, the same
transistor 22 functions as a constant current source. As a result, the
constant drain-to-source current (drive current) Ids, given by the
following formula (I), is supplied to the organic EL element 21 from the
drive transistor 22:

Ids=(1/2)μ(W/L)Cox(Vgs-Vth)2 (1)

[0102] where W is the channel width, L the channel length, and Cox the
gate capacitance per unit area.

[0103] FIG. 7 illustrates the characteristic of the drain-to-source
current Ids of the drive transistor 22 vs. gate-to-source voltage Vgs of
the same transistor 22.

[0104] As illustrated in this characteristic diagram, unless the variation
of the threshold voltage Vth of the drive transistor 22 between the
pixels is corrected, the drain-to-source current Ids appropriate to the
gate-to-source voltage Vgs is Ids1 when the threshold voltage Vth is
Vth1.

[0105] In contrast, when the threshold voltage Vth is Vth2 (Vth2>Vth1),
the drain-to-source current Ids appropriate to the same gate-to-source
voltage Vgs is Ids2 (Ids2<Ids). That is, the drain-to-source current
Ids changes with change in the threshold voltage Vth of the drive
transistor 22 even if the gate-to-source voltage Vgs remains unchanged.

[0106] In the pixel (pixel circuit) 20 configured as described above, on
the other hand, the gate-to-source voltage Vgs of the drive transistor 22
during light emission is Vsig-Vofs+Vth-ΔV as mentioned earlier.
Substituting this into the formula (1), the drain-to-source current Ids
is expressed as follows:

Ids=(1/2)μ(W/L)Cox(Vsig-Vofs-ΔV)2 (2)

[0107] That is, the term of the threshold voltage Vth of the drive
transistor 22 is cancelled. The drain-to-source current Ids supplied from
the drive transistor 22 to the organic EL element 21 is independent of
the threshold voltage Vth of the drive transistor 22. As a result, the
drain-to-source current Ids remains unchanged irrespective of the
variation of the threshold voltage Vth of the drive transistor 22 from
one pixel to another due to the manufacturing process variation or change
over time. This makes it possible to maintain the light emission
brightness of the organic EL element 21 constant.

(Principle of the Mobility Correction)

[0108] A description will be given next of the principle of the mobility
correction of the drive transistor 22.

[0109] FIG. 8 illustrates a characteristic curve comparing a pixel A with
the relatively large mobility μ of the drive transistor 22 and a pixel
B with the relatively small mobility μ of the drive transistor 22. If
the drive transistor 22 includes, for example, a polysilicon thin film
transistor, it is inevitable that the mobility μ varies from one pixel
to another as with the pixels A and B.

[0110] If the video signal voltage Vsig at the same level is, for example,
applied to the pixels A and B when there is a variation in the mobility
μ between the two pixels, there will be a large difference between a
drain-to-source current Ids1' flowing through the pixel A with the large
mobility μ and a drain-to-source current Ids2' flowing through the
pixel B with the small mobility μ, unless the mobility μ is
corrected in one way or another. Thus, the screen uniformity is impaired
in the event of a large difference in the drain-to-source current Ids as
a result of the variation of the mobility μ between the pixels.

[0111] As is clear from the transistor characteristic formula (I) given
above, the larger the mobility p, the larger the drain-to-source current
Ids. Therefore, the larger the mobility p, the larger the negative
feedback amount ΔV. As illustrated in FIG. 8, a feedback amount
ΔV1 of the pixel A with the large mobility μ is larger than a
feedback amount ΔV2 of the pixel B with the small mobility μ.

[0112] For this reason, if the drain-to-source current Ids of the drive
transistor 22 is negatively fed back to the video signal voltage Vsig by
the mobility correction operation, the larger the mobility μ, the
greater the extent to which a negative feedback is applied. This
suppresses the variation of the mobility μ from one pixel to another.

[0113] More specifically, if the pixel A with the large mobility μ is
corrected with the feedback amount ΔV1, the drain-to-source current
Ids declines significantly from Ids1' to Ids1. On the other hand, the
feedback amount ΔV2 of the pixel B with the small mobility μ is
small. Therefore, the drain-to-source current Ids declines merely from
Ids2' to Ids2, which is not a significant drop. As a result, the
drain-to-source current Ids1 of the pixel A becomes almost equal to the
drain-to-source current Ids2 of the pixel B, thus correcting the
variation of the mobility μ from one pixel to another.

[0114] Summing up the above, if the pixels A and B have the different
mobilities μ, the feedback amount ΔV1 of the pixel A with the
large mobility μ is larger than the feedback amount ΔV2 of the
pixel B with the small mobility μ. That is, the larger the mobility p,
the larger the feedback amount ΔV, and the more the drain-to-source
current Ids declines.

[0115] Therefore, the level of the drain-to-source current Ids of the
drive transistor 22 can be made uniform between the pixels with the
different mobilities μ by negatively feeding back the drain-to-source
current Ids of the drive transistor 22 to the video signal voltage Vsig.
This makes it possible to correct the variation of the mobility μ from
one pixel to another.

[0116] Here, a description will be given of the relationship between the
video signal potential (sampling potential) Vsig and drain-to-source
current Ids of the drive transistor 22 in the pixel (pixel circuit) 20
shown in FIG. 2 with reference to FIGS. 9A to 9C. The above relationship
will be described in different cases with and without the threshold and
mobility corrections.

[0117] In FIGS. 9A to 9C, FIG. 9A illustrates the case in which neither
the threshold correction nor the mobility correction is performed. FIG.
9B illustrates the case in which the threshold correction is performed,
but not the mobility correction. FIG. 9C illustrates the case in which
both the threshold and mobility corrections are performed. As illustrated
in FIG. 9A, if neither the threshold correction nor the mobility
correction is performed, there is a large difference in the
drain-to-source current Ids between the pixels A and B as a result of the
variation of the threshold voltage Vth and mobility μ between the two
pixels.

[0118] In contrast, if merely the threshold correction is performed, the
variation of the drain-to-source current Ids can be reduced to some
extent by the threshold correction as illustrated in FIG. 9B. However,
the difference remains in the drain-to-source current Ids between the
pixels A and B caused by the variation of the mobility μ between the
two pixels.

[0119] If both the threshold and mobility corrections are performed, the
difference in the drain-to-source current Ids between the pixels A and B
caused by the variation of the threshold voltage Vth and mobility μ
between the two pixels can be almost completely eliminated as illustrated
in FIG. 9C. This ensures constant brightness of the organic EL element 21
free from variation, thus providing a high-quality on-screen image.

[0120] Further, the following advantageous effects can be achieved by
providing the pixel 20 shown in FIG. 2 with the bootstrapping function
mentioned earlier in addition to the threshold and mobility correction
functions.

[0121] That is, even if the source potential Vs of the drive transistor 22
changes with change in the I-V characteristic of the organic EL element
21 over time, the gate-to-source voltage Vgs of the same transistor 22 is
maintained constant thanks to the bootstrapping action of the holding
capacitance 24. As a result, the current flowing through the organic EL
element 21 remains unchanged. Therefore, the light emission brightness of
the organic EL element 21 is maintained constant. This provides an
on-screen image free from brightness deterioration even in the event of a
change of the I-V characteristic of the organic EL element 21 over time.

[Problems Attributable to Reduced Capacitance Value of the Capacitive
Component of the Organic EL Element]

[0122] As described above, in the organic EL display device 10 having the
threshold and mobility correction functions, as the pixel size becomes
finer as a result of providing a higher definition, the electrodes
forming the organic EL element 21 grow smaller in size. As a result, the
capacitance value of the capacitive component of the same element 21
becomes smaller. This leads to a decline in the write gain of the video
signal voltage Vsig by as much as the decline in the capacitance value of
the capacitive component of the organic EL element 21.

[0123] Here, letting the capacitance value of the EL capacitance 25 be
denoted by Cel and the capacitance value of the holding capacitance 24 by
Cs, the voltage Vgs held by the holding capacitance 24 when the video
signal voltage Vsig is written is expressed as follows:

Vgs=Vsig×{1-Cs/(Cs+Cel)} (3)

[0124] Therefore, the ratio between the voltage Vgs held by the holding
capacitance 24 and the signal voltage Vsig, i.e., a write gain G
(=Vgs/Vsig), can be expressed as follows:

G=1-Cs/(Cs+Cel) (4)

As is clear from this formula (4), if the capacitance value Cel of the
capacitive component of the organic EL element 21 declines, the write
gain G will decline by as much as the decline therein.

[0125] In order to compensate for the decline in the write gain G, an
auxiliary capacitance need merely be attached to the source electrode of
the drive transistor 22. Letting the capacitance value of the auxiliary
capacitance be denoted by Csub, the write gain G can be expressed as
follows:

G=1-Cs/(Cs+Cel+Csub) (5)

[0126] As is clear from the formula (5), the larger the capacitance value
Csub of the auxiliary capacitance to be attached, the closer the write
gain G is to unity. The voltage Vgs close to the video signal voltage
written to the pixel 20 can be held by the holding capacitance 24. This
makes it possible to provide a light emission brightness appropriate to
the video signal voltage written to the pixel 20.

[0127] As is clear from the above description, the write gain G of the
video signal voltage Vsig can be adjusted by adjusting the capacitance
value Csub of the auxiliary capacitance. On the other hand, the drive
transistor 22 differs in size depending upon the light emission color of
the organic EL element 21. Therefore, white balance can be achieved by
adjusting the capacitance value Csub of the auxiliary capacitance
according to the emission color of the organic EL element 21, i.e., the
size of the drive transistor 22.

[0128] On the other hand, letting the drain-to-source current of the drive
transistor 22 be denoted by Ids and the voltage increment corrected by
the mobility correction by ΔV, a mobility correction period t
during which the aforementioned mobility correction is to be performed is
determined as follows:

T=(Cel+Csub)×ΔV/Ids (6)

As is clear from the formula (6), the mobility correction period t can be
adjusted by the capacitance value Csub of the auxiliary capacitance.

[Pixel Configuration Having an Auxiliary Capacitance]

[0129] FIG. 10 is a circuit diagram illustrating the pixel configuration
having an auxiliary capacitance. In FIG. 10, like components are
designated by the same reference numerals as in FIG. 2.

[0130] As illustrated in FIG. 10, the pixel 20 includes the organic EL
element 21 as a light-emitting element. The pixel 20 includes, in
addition to the organic EL element 21, the drive transistor 22, write
transistor 23 and holding capacitance 24. The pixel configured as
described above further includes an auxiliary capacitance 26. The same
capacitance 26 has one of its electrodes connected to the source
electrode of the drive transistor 22 and the other electrode connected to
the common power supply line 34 serving as a fixed potential.

[0131] Here, if the cathode wiring is routed in the TFT layer
(corresponding to a TFT layer 207 in FIGS. 16 to 18) in order to form the
auxiliary capacitance 26, problems occurs such as horizontal crosstalk
which is caused by the limited layout area of the pixel 20 or wiring
resistance in the pixel 20. Horizontal crosstalk occurs due to the wiring
resistance for the following reason.

[0132] If the cathode wiring is routed in the TFT layer, a wiring
resistance R mediates between the cathode electrode of the organic EL
element 21 and common power supply line 34 as illustrated in FIG. 11. As
a result, the cathode potential of the organic EL element 21 fluctuates
synchronously with the variation of the potential of the signal line 33
as illustrated in FIG. 12. When a black window is displayed, for example,
as illustrated in FIG. 13, this fluctuation of the cathode potential is
visually identified as a crosstalk brighter than the regions above and
below the black window on the display screen (horizontal crosstalk).

FEATURES OF THE PRESENT EMBODIMENT

[0133] The present embodiment is, therefore, defined in that the auxiliary
capacitance 26 is formed by positively using auxiliary electrodes 35. The
auxiliary electrodes 35 are each electrically connected to the common
power supply line 34 serving as the cathode electrode of the organic EL
element 21. In the same layer (anode layer) as the anode electrode of the
organic EL element 21, the auxiliary electrodes 35 are at a fixed
potential (cathode potential) and disposed, for example, in rows (one for
each pixel row) for the pixels of the pixel array section 30 arranged in
a matrix form as illustrated in FIG. 14. The other electrode of the
auxiliary capacitance 26 is electrically connected to the auxiliary
electrode 35 (contact is established therebetween) for each of the pixels
20.

[0134] In FIG. 14, the auxiliary electrodes 35 are disposed in rows for
the pixels 20 of the pixel array section 30. However, this is merely an
example. The auxiliary electrodes 35 may be disposed in columns (one for
each pixel column) or in a grid form (one for each pixel row and for each
pixel column) for the pixels 20 of the pixel array section 30. Also in
these cases, contact can be established between the auxiliary electrode
35 and other electrode of the auxiliary capacitance 26 for each of the
pixels 20 as when the auxiliary electrodes 35 are disposed in rows.

(Pixel Layout Structure)

[0135] FIG. 15 is a plan view schematically illustrating a pixel layout
structure of the pixel 20 having the auxiliary capacitance 26.

[0136] As illustrated in FIG. 15, the scan line 31 (one of 31-1 to 31-m)
is disposed along the row (in the row direction of pixels) close to the
upper pixel row. The power supply line 32 (one of 32-1 to 32-m) is
disposed downward from the middle portion. The auxiliary electrode 35 is
disposed along the row above the lower pixel row. Further, the signal
line 33 (one of 33-1 to 33-n) is disposed along the column (in the column
direction of pixels) close to the pixel column on the left.

[0137] The drive transistor 22, write transistor 23 and holding
capacitance 24 are formed in the region between the scan line 31 and
power supply line 32 of the pixel 20. The auxiliary capacitance 26 is
formed in the region between the power supply line 32 and auxiliary
electrode 35 of the pixel 20. Contact (electrical connection) is
established between the other electrode of the auxiliary capacitance 26
and the auxiliary electrode 35 by a contact portion 36 for each of the
pixels. The auxiliary electrode 35 is applied with a fixed potential
(cathode potential) from the common power supply line 34.

[0138] As described above, the auxiliary electrodes 35 are applied with a
fixed potential from the common power supply line 34 serving as the
cathode electrode of the organic EL element 21. The same electrodes 35
are disposed in rows, in columns or in a grid form for the pixels
arranged in a matrix form. For the organic EL display device configured
as described above, specific examples will be described below as to how
to establish contact between the other electrode of the auxiliary
capacitance 26 and the auxiliary electrode 35 for each of the pixels 20
so as to apply a fixed potential to the other electrode of the auxiliary
capacitance 26 and form the auxiliary capacitance 26 for the fixed
potential.

Example 1

[0139] FIG. 16 is a sectional view illustrating the sectional structure of
a pixel 20A according to example 1. The sectional view of FIG. 16 is a
sectional view taken along line A-A of FIG. 15.

[0140] As illustrated in FIG. 16, the pixel 20A has the gate electrode of
the drive transistor 22 formed on a glass substrate 201 as a first wiring
202. A gate insulating film 203 is formed on the first wiring 202. A
semiconductor layer 204 is formed, for example, with polysilicon on the
gate insulating film 203. The same layer 204 forms the source and drain
regions of the drive transistor 22. The power supply line 32 is formed as
a second wiring 206 above the semiconductor layer 204 via an interlayer
insulating film 205.

[0141] Here, the layer which includes the first wiring 202, gate
insulating film 203, semiconductor layer 204 and interlayer insulating
film 205 serves as the TFT layer 207. Further, an insulating planarizing
film 208 and window insulating film 209 are formed successively on the
interlayer insulating film 205 and second wiring 206. The organic EL
element 21 is formed in a concave portion 209A provided in the window
insulating film 209.

[0142] The organic EL element 21 includes an anode electrode 211 made of a
metal or other material formed on the bottom of the concave portion 209A
of the window insulating film 209. The same element 21 further includes
an organic layer (electron transporting layer, light-emitting layer and
hole transporting/injection layer) 212 formed on the anode electrode 211.
The same element 21 still further includes a cathode electrode 213
(common power supply line 34) made, for example, of a transparent
conductive film formed on the organic layer 212 commonly for all the
pixels. Here, the layer which includes the second wiring 206 and
insulating planarizing film 208 serves as an anode layer 210.

[0143] In the organic EL element 21, the organic layer 212 is formed by
depositing the electron transporting layer, light-emitting layer and hole
transporting/injection layer (none of these layers are shown)
successively on the anode electrode 211. As the organic EL element 21 is
current-driven by the drive transistor 22 shown in FIG. 2, a current
flows from the drive transistor 22 to the organic layer 212 via the anode
electrode 211. This causes electrons and holes to recombine in the
light-emitting layer of the organic layer 212, thus causing light to be
emitted.

[0145] In this basic pixel structure, the auxiliary capacitance 26 of the
pixel 20A according to example 1 has the following structure. That is,
one of electrodes 261 is formed with the semiconductor layer 204 made of
polysilicon which forms the source and drain regions of the drive
transistor 22. Other electrode 262 is formed with the same metallic
material and by the same process as for the second wiring 206 so that the
other electrode 262 is opposed to the one of the electrodes 261 via the
interlayer insulating film 205. The auxiliary capacitance 26 is formed
between the opposed regions of the parallel plates of the electrodes 261
and 262.

[0146] Contact is established between the other electrode 262 of the
auxiliary capacitance 26 and the auxiliary electrode 35 by the contact
portion 36. This ensures electrical connection, for each pixel, between
the other electrode 262 of the auxiliary capacitance 26 and the auxiliary
electrodes 35 which are disposed, for example, in rows for the pixels
arranged in a matrix form. As a result, a fixed potential is applied from
the common power supply line 34 via the auxiliary electrodes 35.

[0147] As described above, the auxiliary capacitance 26 is formed with the
electrodes 261 and 262. The one of the electrodes 261 is made of
polysilicon as for the semiconductor layer 204 of the drive transistor
22. The other electrode 262 is made of the same metallic material as for
the second wiring 206. The other electrode 262 is electrically connected,
for each pixel, to the auxiliary electrodes 35 which are disposed, for
example, in rows for the pixels arranged in a matrix form. This makes it
possible to apply a fixed potential to the other electrode 262 of the
auxiliary capacitance 26 without providing any cathode wiring in the TFT
layer 207, thus allowing to form the auxiliary capacitance 26 for the
fixed potential. As a result, problems such as horizontal crosstalk
caused by the limited layout area of the pixel 20 or wiring resistance in
the pixel 20 can be resolved.

[0148] In the case of example 1, the capacitance value of the auxiliary
capacitance 26 is determined by the following, i.e., the area of the
opposed regions of the parallel plates of the electrodes 261 and 262, the
gap between the electrodes 261 and 262 (film thickness of the interlayer
insulating film 205), and the specific inductive capacity of the
insulator (interlayer insulating film 205 in this example) mediating
between the electrodes 261 and 262.

Example 2

[0149] FIG. 17 is a sectional view illustrating the sectional structure of
a pixel 20B according to example 2. In FIG. 17, like components are
designated by the same reference numerals as in FIG. 16. The sectional
view of FIG. 17 is a sectional view taken along line A-A of FIG. 15.

[0150] The pixel 20B according to example 2 has the basic pixel structure
as described in example 1. The auxiliary capacitance 26 of the pixel 20B
has the following structure. That is, the other electrode 262 is formed
first on the glass substrate 201 with the same metallic material and by
the same process as for the first wiring 202. The one of the electrodes
261 is formed via the gate insulating film 203 with polysilicon which
forms the semiconductor layer 204 of the drive transistor 22. The one of
the electrodes 261 is formed where it is opposed to the electrode 262.
The auxiliary capacitance 26 is formed between the opposed regions of the
parallel plates of the electrodes 261 and 262.

[0151] Contact is established between the other electrode 262 of the
auxiliary capacitance 26 and the second wiring 206 by a contact portion
37. Contact is also established between the other electrode 262 of the
auxiliary capacitance 26 and the auxiliary electrode 35 by the contact
portion 36. This ensures electrical connection, for each pixel, between
the other electrode 262 of the auxiliary capacitance 26 and the auxiliary
electrodes 35 which are disposed, for example, in rows for the pixels
arranged in a matrix form. As a result, a fixed potential is applied from
the common power supply line 34 via the auxiliary electrodes 35.

[0152] As described above, the auxiliary capacitance 26 is formed with the
electrodes 261 and 262. The other electrode 262 is made of the same
metallic material as for the first wiring 202. The one of the electrodes
261 is made of polysilicon as for the semiconductor layer 204 of the
drive transistor 22. The other electrode 262 is electrically connected,
for each pixel, to the auxiliary electrodes 35 which are disposed, for
example, in rows for the pixels arranged in a matrix form. This makes it
possible to apply a fixed potential to the other electrode 262 of the
auxiliary capacitance 26 without providing any cathode wiring in the TFT
layer 207, thus allowing to form the auxiliary capacitance 26 for the
fixed potential. As a result, problems such as horizontal crosstalk
caused by the limited layout area of the pixel 20 or wiring resistance in
the pixel 20 can be resolved.

[0153] In the case of example 2, the capacitance value of the auxiliary
capacitance 26 is determined by the following, i.e., the area of the
opposed regions of the parallel plates of the electrodes 261 and 262, the
gap between the electrodes 261 and 262 (film thickness of the gate
insulating film 203), and the specific inductive capacity of the
insulator (gate insulating film 203 in this example) mediating between
the electrodes 261 and 262.

[0154] Here, examples 1 and 2 are compared. Assuming that both the
specific inductive capacity and area of the opposed regions of the
parallel plates are the same, the following can be said. That is, the
gate insulating film 203 is typically thinner than the interlayer
insulating film 205. Therefore, the gap between the parallel plates can
be made smaller in example 2 than in example 1. As a result, the
capacitance value of the auxiliary capacitance 26 can be set larger in
example 2 than in example 1.

[0155] Conversely, example 1 has an advantage over example 2 in that leak
caused by interlayer shorting is less likely to occur because the
interlayer insulating film 205 is thicker than the gate insulating film
203.

Example 3

[0156] FIG. 18 is a sectional view illustrating the sectional structure of
a pixel 20C according to example 3. In FIG. 18, like components are
designated by the same reference numerals as in FIGS. 16 and 17. The
sectional view of FIG. 18 is a sectional view taken along line A-A of
FIG. 15.

[0157] The pixel 20C according to example 3 has the basic pixel structure
as described in example 1. The auxiliary capacitance 26 of the pixel 20C
has the following structure. That is, an other first electrode 262A is
formed first on the glass substrate 201 with the same metallic material
and by the same process as for the first wiring 202. The one of the
electrodes 261 is formed via the gate insulating film 203 with
polysilicon which forms the semiconductor layer 204 of the drive
transistor 22. The one of the electrodes 261 is formed where it is
opposed to the electrode 262. Further, an other second electrode 262B is
formed with the same metallic material and by the same process as for the
second wiring 206 so that it is opposed to the electrode 261 via the
interlayer insulating film 205. The auxiliary capacitance 26 is formed
electrically in parallel between the opposed regions of the parallel
plates of the electrodes 262A, 261 and 262B.

[0158] Contact is established between the other first electrode 262A of
the auxiliary capacitance 26 and the other second electrode 262B by the
contact portion 37. Contact is also established between the other first
electrode 262A of the auxiliary capacitance 26 and the auxiliary
electrode 35 by the contact portion 36. This ensures electrical
connection, for each pixel, between the other first and second electrodes
262A and 262B of the auxiliary capacitance 26 and the auxiliary
electrodes 35 which are disposed, for example, in rows for the pixels
arranged in a matrix form. As a result, a fixed potential is applied from
the common power supply line 34 via the auxiliary electrodes 35. Further,
the capacitance formed between the electrodes 262A and 261 and that
formed between the electrodes 262B and 261 are connected electrically in
parallel so that the auxiliary capacitance 26 is formed as the combined
capacitance of the two capacitances.

[0159] As described above, the auxiliary capacitance 26 is formed with the
other electrodes 262A and 262B and one of electrodes 261. The other
electrodes 262A and 262B are respectively made of the same metallic
materials as for the first and second wirings 202 and 206. The one of
electrodes 261 is made of polysilicon as for the semiconductor layer 204
of the drive transistor 22. The other electrodes 262A and 262B are
electrically connected, for each pixel, to the auxiliary electrodes 35
which are disposed, for example, in rows for the pixels arranged in a
matrix form. This makes it possible to apply a fixed potential to the
other electrodes 262A and 262B of the auxiliary capacitance 26 without
providing any cathode wiring in the TFT layer 207, thus allowing to form
the auxiliary capacitance 26 for the fixed potential. As a result,
problems such as horizontal crosstalk caused by the limited layout area
of the pixel 20 or wiring resistance in the pixel 20 can be resolved.

[0160] In particular, a capacitance is formed between the other first
electrode 262A and one of the electrodes 261 and another between the one
of the electrodes 261 and other second electrode 262B. Therefore,
assuming that the capacitance values in examples 1 and 2 are the same,
the auxiliary capacitance 26 having a capacitance value roughly twice as
large as that in examples 1 and 2 can be formed. In other words, if the
auxiliary capacitance 26 need merely have more or less the same
capacitance value as in examples 1 and 2, the electrodes 261, 262A and
262B forming the auxiliary capacitance 26 can be reduced in size. As a
result, the auxiliary capacitance 26 can be formed in the pixel 20
without increasing the size of the pixel 20C as compared to examples 1
and 2.

[0161] In the case of example 3, the capacitance value of the auxiliary
capacitance 26 is determined by the combined capacitance value of the two
capacitances. One of the capacitances is determined by the area of the
opposed regions of the parallel plates of the one of the electrodes 261
and other first electrode 262A, the distance between the electrodes 261
and 262A, and the specific inductive capacity of the insulator (gate
insulating film 203 in this example) mediating between the electrodes 261
and 262A. The other capacitance is determined by the area of the opposed
regions of the parallel plates of the one of the electrodes 261 and other
second electrode 262B, the distance between the electrodes 261 and 262B,
and the specific inductive capacity of the insulator (interlayer
insulating film 205 in this example) mediating between the electrodes 261
and 262B.

ADVANTAGEOUS EFFECTS OF THE PRESENT EMBODIMENT

[0162] As described above, the pixels 20 of the organic EL display device
each have the auxiliary capacitance 26 to secure a sufficient write gain
of the video signal. In this organic EL display device, the other
electrode or electrodes 262 (262A and 262B) of the auxiliary capacitance
26 are connected, for each of the pixels 20, to the auxiliary electrodes
35 which are disposed in rows, in columns or in a grid form for the
pixels arranged in a matrix form and which are applied with a fixed
potential. This makes it possible to apply a fixed potential to the other
electrodes 262 without providing any cathode wiring in the TFT layer 207,
thus allowing to form the auxiliary capacitance 26 for the fixed
potential while at the same time suppressing the wiring resistance. As a
result, horizontal crosstalk caused by the wiring resistance can be
suppressed, thus providing improved on-screen image quality.

[0163] In the above embodiment, a description was given taking, as an
example, the case in which the present invention was applied to an
organic EL display device using organic EL elements as electro-optical
elements of the pixel circuits. However, the embodiment of the present
invention is not limited to this application example, but applicable to
display devices in general using current-driven electro-optical elements
(light-emitting elements) whose light emission brightness changes with
change in current flowing through the elements.

Application Examples

[0164] The display device according to the embodiment of the present
invention described above is applicable as a display device of electronic
equipment across all fields including those shown in FIGS. 19 to 23,
namely, a digital camera, laptop personal computer, mobile terminal
device such as mobile phone and video camcorder. These pieces of
equipment are designed to display an image or video of a video signal fed
to or generated inside the electronic equipment.

[0165] As described above, if used as a display device of electronic
equipment across all fields, the display device according to the
embodiment of the present invention can, as is clear from the
aforementioned embodiment, prevent horizontal crosstalk caused by the
wiring resistance because contact is established, for each of the pixels
20, between the other electrode of the auxiliary capacitance 26 and the
auxiliary electrodes 35 which are disposed in rows, in columns or in a
grid form for the pixels arranged in a matrix form. As a result, the
display device according to the embodiment of the present invention
provides excellent on-screen image quality in all kinds of electronic
equipment.

[0166] It should be noted that the display device according to the
embodiment of the present invention includes that in a modular form
having a sealed configuration. Such a display device corresponds to a
display module formed by attaching an opposed section made, for example,
of transparent glass to the pixel array section 30. The aforementioned
light-shielding film may be provided on the transparent opposed section,
in addition to films such as color filter and protective film. It should
also be noted that a circuit section, FPC (flexible printed circuit) or
other circuitry, adapted to allow exchange of signals or other
information between external equipment and the pixel array section, may
be provided on the display module.

[0167] Specific examples of electronic equipment to which the embodiment
of the present invention is applied will be described below.

[0168] FIG. 19 is a perspective view illustrating a television set to
which the embodiment of the present invention is applied. The television
set according to the present application example includes a video display
screen section 101 made up, for example, of a front panel 102, filter
glass 103 and other parts. The television set is manufactured by using
the display device according to the embodiment of the present invention
as the video display screen section 101.

[0169] FIGS. 20A and 20B are perspective views illustrating a digital
camera to which the embodiment of the present invention is applied. FIG.
20A is a perspective view of the digital camera as seen from the front,
and FIG. 20B is a perspective view thereof as seen from the rear. The
digital camera according to the present application example includes a
flash-emitting section 111, display section 112, menu switch 113, shutter
button 114 and other parts. The digital camera is manufactured by using
the display device according to the embodiment of the present invention
as the display section 112.

[0170] FIG. 21 is a perspective view illustrating a laptop personal
computer to which the embodiment of the present invention is applied. The
laptop personal computer according to the present application example
includes, in a main body 121, a keyboard 122 adapted to be manipulated
for entry of text or other information, a display section 123 adapted to
display an image, and other parts. The laptop personal computer is
manufactured by using the display device according to the embodiment of
the present invention as the display section 123.

[0171]FIG. 22 is a perspective view illustrating a video camcorder to
which the embodiment of the present invention is applied. The video
camcorder according to the present application example includes a main
body section 131, lens 132 provided on the front-facing side surface to
image the subject, imaging start/stop switch 133, display section 134 and
other parts. The video camcorder is manufactured by using the display
device according to the embodiment of the present invention as the
display section 134.

[0172] FIGS. 23A to 23G are perspective views illustrating a mobile
terminal device such as mobile phone to which the embodiment of the
present invention is applied. FIG. 23A is a front view of the mobile
phone in an open position. FIG. 23B is a side view thereof. FIG. 23C is a
front view of the mobile phone in a closed position. FIG. 23D is a left
side view. FIG. 23E is a right side view. FIG. 23F is a top view. FIG.
23G is a bottom view. The mobile phone according to the present
application example includes an upper enclosure 141, lower enclosure 142,
connecting section (hinge section in this example) 143, display 144,
subdisplay 145, picture light 146, camera 147 and other parts. The mobile
phone is manufactured by using the display device according to the
embodiment of the present invention as the display 144 and subdisplay
145.

[0173] It should be understood by those skilled in the art that various
modifications, combinations, sub-combinations and alterations may occur
depending on design requirements and other factor in so far as they are
within the scope of the appended claims or the equivalents thereof.